Method and device for microwave plasma deposition of a coating on a thermoplastic container surface

ABSTRACT

The invention concerns the deposition of a coating on a thermoplastic container surface ( 3 ) using low pressure plasma by excitation of a precursor gas with UHF electromagnetic waves in a circular shaped vacuum cavity ( 1 ) receiving the container. It consists in dimensioning the cavity ( 1 ) with respect to the frequency of the UHF electromagnetic waves so as to obtain a coupling mode generating several electromagnetic fields inside the cavity. In particular a TM 120 coupling mode is provided which generates two central fields ( 4   A   , 4   B ) inside the cavity, whereby two containers ( 3 ) can be simultaneously treated in said cavity ( 1 ).

The present invention relates to improvements made in the field of thedeposition of a coating on a face of a thermoplastic container using alow-pressure plasma by excitation of a precursor gas by UHFelectromagnetic waves in a circular vacuum chamber (or reactor)containing said container.

The invention applies more particularly to a deposition of a barrierlayer inside bottles or pots made of a thermoplastic, such as PET, so asto improve the gas barrier properties, with regard to internal gases orexternal gases, and possibly to improve the isolation from the outsideof the product with which said bottles or pots are filled.

A device for depositing such a coating using a low-pressure plasma byexcitation of a precursor gas by means of UHF electromagnetic waves hasbeen described and illustrated in document FR 2 799 994. The UHFgenerator is connected to the chamber via a UHF waveguide, which runsinto a window of the side wall of the chamber, with a TM 020 couplingmode that generates an axial central field in the chamber. To undergothe treatment envisioned, the container to be treated is thereforeplaced at the center of the chamber in a quartz envelope coaxial withthe chamber.

In an industrial production machine, several (typically 20) devices arejoined together on a rotating structure capable of treating about 10,000bottles per hour.

These machines are satisfactory as regards the quality of the containersobtained.

However, there is a keen interest for users to have a higher treatmentrate.

An increase in rate could admittedly be achieved by installing a largernumber of devices on the rotating structure. However, this increase inthe number of devices could be rendered possible only by increasing thedimensions of the rotating structure. This would result in a larger,heavier and therefore more expensive machine, which is unacceptable.

Likewise, the operation of a second machine running in parallel with thefirst would admittedly allow the rate to be doubled, but here again thiswould result in bigger space required and a higher cost, which areunacceptable.

The object of the invention is consequently to improve the existingdevices, leading to a machine that is more efficient in terms ofproduction while still maintaining acceptable size and an acceptablecost.

For this purpose, according to a first of its aspects, the inventionproposes a method for depositing a coating on one face of a containermade of a thermoplastic using a low-pressure plasma by excitation of aprecursor gas by UHF electromagnetic waves in a circular vacuum chambercontaining said container, which method is, according to the invention,characterized in that the chamber is sized in relation to the frequencyof the UHF electromagnetic waves so as to obtain a coupling mode thatgenerates several electromagnetic fields inside the chamber, whereby itis possible for several respective containers to be simultaneouslytreated in the same chamber. In a preferred embodiment, a TM 120coupling mode is established, which generates two symmetrical fieldsinside the chamber, these fields themselves having two separate energyregions, whereby it is possible for two containers to be simultaneouslytreated in said chamber, this method offering the advantage of beingable to be implemented in a simple manner in conjunction withcommercially available magnetrons operating at a frequency of 2.455 GHz.

Thus, thanks to the method of the invention, it is possible to doublethe container treatment rate solely by an arrangement of the currentlyknown means, and therefore relatively inexpensively.

According to a second of its aspects, the invention proposes, forimplementing the aforementioned method, a device for depositing acoating on one face of a container made of a thermoplastic using alow-pressure plasma by excitation of a precursor gas by UHFelectromagnetic waves in a circular vacuum chamber containing saidcontainer, which device comprises a UHF wave generator and a UHFwaveguide for connecting said generator to a window in the side wall ofthe chamber, which device, being designed in accordance with theinvention, is characterized in that the chamber is sized in relation tothe frequency of the UHF electromagnetic waves in order to establish aTM 120 coupling mode that generates two symmetrical fields in thecavity, these fields themselves having two separate energy regions,whereby it is possible for two containers to be simultaneously treatedin said chamber.

In a preferred practical embodiment, the generator emits anelectromagnetic wave having a frequency f=2.455 GHz and the diameter ofthe chamber is approximately 273 mm. The generator is a magnetroncommonly employed in other fields. As regards the diameter of thechamber, this is perfectly compatible with the structures of the currentmachines. It therefore proves possible, through a simple modification ofthe current machines, to double the treatment capacity of the machines,since the diameter of the chamber permits simultaneous treatment of twobottles of half-liter or smaller size placed side by side in therespective two central fields.

Particularly advantageously, the chamber contains two quartz envelopesplaced respectively so as to be approximately coaxial with the twoabovementioned symmetrical fields, the chamber includes a single windowfor injecting the UHF waves, the window being located symmetricallystraddling the plane of symmetry on each side of which the two centralfields are situated, and a single cover for closing off the chamber isequipped with a single coupler for connection to a vacuum source, whichis divided into two in order to be connected to the abovementioned tworespective envelopes, with two precursor gas injectors that areconnected to a single precursor gas source and with two support meansfor the two respective containers, in such a way that using theprovisions of the invention does not entail having to duplicate thenecessary equipment (such as pressure sensors on the inside and theoutside of the container).

It is advantageous for the device to also include bottom and top plates,the position of each of which can be adjusted, these plates beingsuitable for acting on the respective return fields so as to refine thecoupling according to the various types of container that can betreated.

Within the context of the specifically preferred application envisioned,the device is designed for coating the inside of containers and for thispurpose, the precursor gas injectors are designed to sit inside therespective containers when the latter are supported by the support meansin the envelopes.

The invention will be more clearly understood on reading the detaileddescription that follows of a preferred embodiment, most particularlysuitable for coating the inside of containers, and given solely by wayof nonlimiting example. In this description, reference will be made tothe appended drawings in which:

FIG. 1 is a diagram illustrating the conditions under which the methodof the invention are carried out; and

FIG. 2 is a schematic representation of a device for implementing themethod of the invention.

FIG. 1 shows schematically a chamber 1 having the general shape of acylinder of revolution, which has, in its side wall, an opening 2through which a waveguide connected to a UHF electromagnetic wavegenerator (not shown) enters.

The UHF generator is a magnetron operating at a frequency of 2.455 GHz.

In order to be able to treat several containers 3 (the two containers 3are shown schematically in dotted lines) simultaneously in the chamber1, the dimensions of the chamber are chosen in relation to the frequencyof the UHF electromagnetic waves, so as to obtain a coupling mode thatgenerates several electromagnetic fields inside the chamber, eachcontainer 3 being placed coaxially in a respective field.

In one practical embodiment of this arrangement, a TM 120 coupling modeis established, which generates two symmetrical electromagnetic fields,these fields themselves having two separate energy regions, namely twocentral fields 4 _(A) and 4 _(B) and two return fields 5 _(A), 5 _(B)located peripherally, in the shape of haricot beans, facing the innerfields, as illustrated in FIG. 1. The two containers 3 to be treated areplaced coaxially in the respective central fields 4 _(A), 4 _(B). It isalso desirable for the positionally adjustable bottom 17 i and top 17_(B) plates (visible in FIG. 2) to act on the return field 5 _(A), 5_(B) in order to refine the coupling to the reactor according to thevarious types of container 3 capable of being treated.

Under these conditions, the cut-off wavelength is given by:$\lambda_{c} = \frac{2\pi\quad R}{U_{12}}$where R is the radius of the chamber and U₁₂ characterizing the T₁₂₀mode has a value U₁₂=7.0156.

The cut-off wavelength λ_(c) has a value close to (but slightly greaterthan) the wavelength λ of the generator:$\lambda = {\frac{v}{f} = {\frac{3 \times 10^{8}}{2\text{,}455 \times 10^{9}} = {{12\text{,}22\quad{cm}}->{\lambda_{c}\quad{\# 12}\text{,}225\quad{cm}}}}}$

The radius R of the chamber is:$R = {\frac{\lambda_{c} \times U_{12}}{2\pi} = {\frac{12.225 \times 7.0156}{2\pi} = {13.65\quad{cm}}}}$

The chamber therefore has to have a diameter of approximately 273 mm.

The diameter of the chamber 1 thus formed allows two containers, such astwo bottles 50 cl in volume or less to be treated simultaneously. Byoperating in this manner, the treatment capacity of each chamber isdoubled. This makes it possible to meet, very favorably, therequirements of users while still maintaining a chamber havingdimensions compatible with the rotating structures currently used. Inother words, the arrangements according to the invention may beimplemented without it being necessary to redesign the entire rotatingstructure.

FIG. 2 illustrates, in side view, a container treatment device producedaround the chamber 1 shown schematically in FIG. 1.

The device shown in FIG. 2, denoted in its entirety by the numericalreference 6, includes a cavity (or reactor) 1 which is a cylinder ofrevolution having a diameter of approximately 273 mm. The side wall ofthe chamber 1 has, approximately at mid-height, an opening 2 throughwhich a waveguide (not shown) enters, said waveguide being connected toa UHF generator 7 (largely concealed by the chamber), for example formedby a magnetron, capable of operating at the frequency of 2.455 GHz. Thisdevice generates a TM 120 coupling mode with two central fields asillustrated in FIG. 1, these two central fields being centered on theaxes A and B shown in FIG. 2.

The arrangement of the device is transposed from that of a device fortreating a single container, as described and shown in particular in theabovementioned document FR-A-2 799 994. In particular, two quartzenvelopes 8 are placed in the chamber, coaxially with the axes A and B,and inside which envelopes the two respective containers 3 are placed.These two envelopes 8 are mounted in a vacuum-tight manner (with seals18) in the chamber, each envelope defining a cavity of small volume inwhich the container may be placed and making it easier to create thevacuum required by the generation of the plasma necessary for depositingthe coating in each container.

However, one advantage of the arrangement adopted lies in the fact thatthe device still requires just one superstructure. In other words, thesingle cover 9 for the chamber incorporates, on the one hand, themembers 10 for supporting the two containers 3 and, on the other hand,the connection means needed for creating the vacuum in the chamber andfor injecting the precursor gas needed to form the plasma, and also theinternal pressure sensor and the external pressure sensor.

Thus, the cover 9 is provided with an enclosure 10 connected (via aline, not shown in FIG. 2) to a vacuum source, which enclosure 10extends above the two containers 3 and is in communication at 11 withthe inside of the containers. In the example shown, the passage 11 iscombined with the means 12 for supporting each container 3.

According to the preferred way of implementing the invention for coatingthe inside of containers, each passage 11 is penetrated coaxially by aprecursor gas injector 13 which sits inside the corresponding container3. The two injectors 13 may be connected, on the outside of the cover 9,to a single line 14 for connection to a source (not visible in thefigures) of precursor gas.

The cover 9 may furthermore be equipped with a valve 15 for bringing theenclosure 10 into communication with ducts 16, either for bringing theinside of the containers 3 into communication with the inside of theenvelopes 8 while the vacuum is being created, or for isolating them soas to be able to create differential pressure conditions suitable forgenerating a plasma in the containers.

In total, the provisions according to the invention, which consist inestablishing a TM 120 coupling mode in order to be able to treat twocontainers simultaneously, prove to be beneficial in the sense thatwhile admittedly it is necessary to duplicate all the elements thatcooperate directly with the two containers (i.e. two quartz envelopes,two injectors, two support means, two vacuum orifices), the remainder ofthe installation nonetheless remains common (i.e. one single cavity, onesingle UHF generator, one single source and one single feed for thevacuum, one single source and one single feed for the precursor gas, onesingle internal pressure sensor, one single external pressure sensor,one single cover and therefore one single mechanism for actuating(lowering/raising) the cover, a single mechanism for gripping thecontainers, in order to hold them in place and for removing them, etc.).

In addition, since there is only one cover 9, there remain only singlemeans for actuating said cover, in order to close/open the chamber 1,such as those mentioned in document FR-A-2 799 994.

In general, the arrangement of the chamber 1 must comply with thesymmetry provided by the two central fields 4 _(A), 4 _(B). Inparticular, the window 2 through which the waveguide enters the chamber1 is placed on the axis between the two central fields 4 _(A), 4 _(B),as may be seen in FIGS. 1 and 2. Likewise, columns for supporting theimpedance-matching plates 17 _(i), 17 _(s) for the respective externalfields 5 _(A), 5 _(B) (these columns not being shown in order not toclutter up the drawings; see for example document FR-A-2 792 854) mustbe placed symmetrically on either side of the window 2.

1. A method for depositing a coating on one face of a container made ofa thermoplastic using a low-pressure plasma by excitation of a precursorgas by UHF electromagnetic waves in a circular vacuum chamber containingsaid container, wherein the chamber is sized in relation to thefrequency of the UHF electromagnetic waves so as to obtain a couplingmode that generates several electromagnetic fields inside the chamber,whereby it is possible for several respective containers to besimultaneously treated in the same chamber.
 2. The method as claimed inclaim 1, wherein a TM 120 coupling mode is established, which generatestwo central fields (4 _(A), 4 _(B)) inside the chamber, whereby twocontainers can be simultaneously treated in said chamber.
 3. A devicefor depositing a coating on one face of a container made of athermoplastic using a low-pressure plasma by excitation of a precursorgas by UHF electromagnetic waves in a circular vacuum chamber containingsaid container, which device comprises a UHF wave generator and a UHFwaveguide for connecting said generator to a window of the side wall ofthe chamber, wherein the chamber is sized in relation to the frequencyof the UHF electromagnetic waves in order to establish a TM 120 couplingmode that generates two central fields (4 _(A), 4 _(B)) in the cavity,whereby it is possible for two containers to be simultaneously treatedin said chamber.
 4. The device as claimed in claim 3, wherein thegenerator emits an electromagnetic wave having a frequency f=2.455 GHzand in that the diameter of the chamber is approximately 273 mm.
 5. Thedevice as claimed in claim 3, wherein the chamber contains two quartzenvelopes mounted in a vacuum-tight manner in the chamber and placedrespectively so as to be approximately coaxial with the two centralfields (4 _(A), 4 _(B)), in that the chamber includes a single windowfor injecting the UHF waves, the window being located along the axis ofsymmetry of the two central fields (4 _(A), 4 _(B)), and in that asingle cover for closing off the chamber is equipped with a singlecoupler for connection to a vacuum source, which is divided into two (at11) in order to be connected to the abovementioned two respectiveenvelopes, with two precursor gas injectors that are connected to asingle precursor gas source and with two support means for the tworespective containers.
 6. The device as claimed in claim 5, wherein itincludes positionally adjustable bottom (17 i) and top (17 s) platessuitable for acting on the respective return fields (5 _(A), 5 _(B)) soas to refine the coupling according to the various types of containerthat can be treated.
 7. The device as claimed in claim 5, wherein it isdesigned for coating the inside of containers and in that for thispurpose, the precursor gas injectors are designed to sit inside therespective containers when the latter are supported by support means inthe envelopes.